Testing

Mockery provides the capacity for stubbing out functions and for verifying
function calls during testing.

Stubbing

Mockery's stub function will let you stub out a function with another
function or simply a return value. Note that if you choose to replace the
function with another function, the signatures of the two functions should be
compatible.

Examples

g=function(y) yf=function(x) g(x) +1
test_that('demonstrate stubbing', {
# replaces 'g' with a function that always returns 100# but only when called from f
stub(f, 'g', 100)
# this can also be written
stub(f, 'g', function(...) 100)
expect_equal(f(1), 101)
})

Stubbing works with classes of all descriptions and namespaced functions:

# this stubs the 'request_perform' function, but only# for httr::get, and only when it is called from within this# test function
stub(httr::GET, 'request_perform', 'some html')
# it is also possible to stub out a namespaced function call
stub(some_function, 'namespace::function', 'some return value')

This also works with R6 classes and methods.

Depth

It's possible to specify the depth of stubbing. This is useful if you
want to stub a function that isn't called directly by the function you call in
your test, but is instead called by a function that that function calls.

In the example below, the function g is both called directly from r, which
we call from the test, and from f, which r calls. By specifying a depth of
2, we tell mockery to stub g in both places.

For more examples, please see the test code contained in this repository.

Comparison to with_mock

Mockery's stub function has similar functionality to testthat's with_mock.

There are several use cases in which mockery's stub function will work, but
testthat's with_mock will not.

First, unlike with_mock, it seamlessly allows for mocking out primitives.

Second, it is easy to stub out functions from base R packages with mockery's stub.
Because of how with_mock works, you can get into trouble if you mock such functions
that the JIT compiler might try to use. These kinds of problems are avoided by stub's
design. As of version 2.0.0 of testthat, it will be impossible to mock functions from
base R packages with_mock.

The functionality of stub is just slightly different than that of with_mock. Instead
of mocking out the object of interest for the duration of some code block, it mocks it
out only when it is called from a specified function.

Mocking

Mock objects allow you to specify the behavior of the function you've stubbed
out while also verifying how that function was used.